Since heating of closed spaces began, mankind has been concerned with obtaining as much heat from a given amount of fuel as possible. During the period of plentiful oil and gas supplies not only the heating of enclosures such as buildings and homes but also the use of heat in the curing of crops and the like was accomplished through the use of these fossil fuels. As the cost of fuels has escalated, however, alternative sources of energy have been explored.
In large commercial operations, however, such as the critically controlled temperatures for curing tobacco, drying grain and the like, wood-fired and solar systems have not been thought practical on a large scale because of the inefficiency of such system as well as, in the case of the wood and organic material fired units, the pollution created as a by-product of the combustion process.